Dehydroxylative radical N-glycosylation of heterocycles with 1-hydroxycarbohydrates enabled by copper metallaphotoredox catalysis

N-Glycosylated heterocycles play important roles in biological systems and drug development. The synthesis of these compounds heavily relies on ionic N-glycosylation, which is usually constrained by factors such as labile glycosyl donors, precious metal catalysts, and stringent conditions. Herein, we report a dehydroxylative radical method for synthesizing N-glycosides by leveraging copper metallaphotoredox catalysis, in which stable and readily available 1-hydroxy carbohydrates are activated for direct N-glycosylation. Our method employs inexpensive photo- and copper- catalysts and can tolerate some extent of water. The reaction exhibits a broad substrate scope, encompassing 76 examples, and demonstrates high stereoselectivity, favoring 1,2-trans selectivity for furanoses and α-selectivity for pyranoses. It also exhibits high site-selectivity for substrates containing multiple N-atoms. The synthetic utility is showcased through the late-stage functionalization of bioactive compounds and pharmaceuticals like Olaparib, Axitinib, and Metaxalone. Mechanistic studies prove the presence of glycosyl radicals and the importance of copper metallaphotoredox catalysis.

The development of mechanistically distinct glycosylation methods may provide opportunities to address some of the aforementioned limitations.Considering the mild conditions, great functionality tolerance, and unique stereoselectivity, a glycosyl radical-based approach would be intriguing [23][24][25] .However, great challenges exist for realizing such an N-glycosylation, including common issues such as stereoselectivity and regioselectivity, as well as the tendency of glycosyl radicals to undergo homocoupling 26,27 , oxidation 28 or reduction 29 .Consequently, while the synthesis of C-glycosides [23][24][25] and S-glycosides [30][31][32] has been well-established, research on corresponding N-glycosylation remains virtually untouched.Recently, Niu et al. demonstrated the possibility of radical N-glycosylation by utilizing allyl glycosyl sulfones as glycosyl donors 28 .However, their investigation was limited to only two substrates, and the formation of the C-N bond still followed an ionic S N 2 process via glycosyl iodides (Fig. 1c) 28 .
Inspired by recent advances in copper-catalyzed C-N coupling reaction [33][34][35][36][37][38][39] and dehydroxylative coupling reactions 40 , specifically employing metallaphotoredox catalysis 41 , we sought to develop a copper-catalyzed, dehydroxylative radical N-glycosylation as an alternative to conventional ionic approaches.In our working hypothesis (Fig. 1d), we envisioned that a glycosyl radical could be generated from a 1-hydroxycarbohydrate after proper activation 40 under mildly photoredox conditions [23][24][25]42 . It an then capture the copper (II)-amido complex efficiently 43 .Subsequently, the resulting high-valent Cu(III) species can undergo facile reductive elimination to yield the desired N-glycosides 44,45 .We report herein the successful realization of this hypothesis (Fig. 1c).Merits of this work include i) mechanistically distinct, dehydroxylative radical N-glycosylation; ii) readily available and stable 1-hydroxycarbohydrates as the glycosyl donors and inexpensive copper catalyst; iii) highly diastereoselective and regioselective for a broad substrate scope.

Results and discussion
Though alkyl halides and carboxylic acids have been extensively studied in C-N coupling reactions [33][34][35][36][37][38][39] , the corresponding glycosyl counterparts are either unstable or difficult to prepare 46 .Therefore, our objective was to directly activate 1-hydroxycarbohydrates to form glycosyl radicals.Following a preliminary investigation of hemioxalate 47 , dihydropyridine (DHP) ester 48,49 and xanthate salt 50 activation techniques (see section 2.4 in Supplementary Information), we decided to use the N-heterocyclic carbene (NHC) activation method developed by MacMillan and co-workers (Supplementary Table 13) [51][52][53] .This method does not require isolation or workup to prepare the NHC-adduct, making it the preferred choice.

Table 1 | Optimization of the Reaction Conditions a
Entry Variation from theoptimal conditions Yield(%) [c] Entry Variation from the optimal conditions Yield(%) [c]  optimized conditions, suggesting the unique role of glycosyl radicals in this transformation.It is noteworthy that while 1-hydroxy carbohydrates have been utilized as glycosyl donors for O- 55 and C-glycosylation 51 , we demonstrate the successful application of 1-hydroxy carbohydrates for N-glycosylation in this work.
To demonstrate the applicability of this N-glycosylation protocol, we sought to functionalize a wide variety of structurally complex molecules in a late-stage fashion.Marketed anticancer drugs, including Olaparib (9a), Tegafur (9b), and Axitinib (9d), underwent successful conversion to the corresponding N-glycosides with yields ranging from 47% to 58% as the exclusive α-isomer.Metaxalone, a muscle relaxant (9c) and Losartan (9e), an anti-hypertension drug, are also viable substrates.Interestingly, the latter compound demonstrated that the presence of a free alcohol group does not affect the reaction efficiency.Finally, the natural product rutecarpine, derived from the traditional Chinese medicine fructus evodiae, underwent efficient Nglycosylation, resulting in the formation of (9f) with a 53% yield.
To understand the reaction pathway, we have conducted a series of mechanistic studies (see section 2.5 in Supplementary Information).The reaction was completely shut down by the addition of TEMPO, thus suggesting a possible radical pathway (Fig. 4a).When 10 with an allyl group at O-2 position was used, diastereomers of 11 were obtained in 38% yield, which presumably arises from 5-exo-trig cyclization of the radical intermediate (Fig. 4b).This radical clock reaction clearly proved the involvement of glycosyl radical intermediates.The reaction cannot occur without photosentizer or light (Entries 3, 5, Table 1).Furthermore, the Stern-Volmer experiments showed that NHCalcohol adduct quenched the excited state photocatalyst (PC*) significantly faster than other reagents such as an indazole (Fig. 4c).These experiments, along with the control experiments listed in the reaction optimization section (Table 1), suggested glycosyl radicals are likely generated by photoactivation.
We next conducted UV-vis absorption spectroscopy and cyclic voltammetry experiments (see section 2.5.3-2.5.4 in Supplementary Information) to get some insight on the oxidation of Cu (I) to Cu (II) in the copper catalytic cycle (Fig. 4d).According to the UV/Vis studies, reaction of LnCu(I) with t-BuOOH generated a new copper species (i→ii), which has a nearly identical profile as LnCu(II) (iii).The oxidation of LnCu(I)-amido to LnCu(II)-amido (iv→v) is also feasible, probably easier than the oxidation of LnCu(I) to LnCu(II), as shown by the CV experiment (Supplementary Fig. 7).
On the basis of the conducted experiments, we proposed a plausible mechanism as illustrated in Fig. 5.The reaction is initiated by the Single Electron Transfer (SET) oxidation of the NHC adduct I with 4CzIPN 51 , resulting in the formation of glycosyl radical II.In the copper catalytic cycle, the ligand change reaction of the copper catalyst with an N-heterocycle (e.g., 1b) generates copper (I)-amido complex III.This species is then oxidized by the oxidant (tBuOOH), leading to the formation of copper (II)-amido species IV.The glycosyl radical is then captured by IV, yielding copper (III) complex V 43 , which gives the desired product through reductive elimination 44,45 , thus regenerating the copper catalyst and closing the copper catalytic cycle.The photoredox cycle is closed by the oxidation of the reduced form of the photocacalyst (PC-) with the oxidant (tBuOOH), followed by sensitization.
In summary, we have developed a dehydroxylative radical method for synthesizing N-glycosides by leveraging copper metallaphotoredox catalysis, in which stable and readily available 1-hydroxy carbohydrates are directly activated for N-glycosylation.Complementing with the well-established ionic approaches, our method employs inexpensive photo-and copper-catalysts and can tolerate some extent of water.The reaction exhibits a broad substrate scope, encompassing 76 examples, and demonstrates high stereoselectivity, favoring 1,2-trans selectivity for furanoses and α-selectivity for pyranoses.It also exhibits high site-selectivity for substrates containing multiple N-atoms.The synthetic utility was showcased through the late-stage functionalization of bioactive compounds and pharmaceuticals like Olaparib, Axitinib, and Metaxalone.The presence of glycosyl radicals was confirmed through radical suppressing reactions and a radical clock reaction.Additionally, the importance of copper metallaphotoredox catalysis was demonstrated through control experiments and various spectroscopic studies, such as UV-vis experiments.Though limitations, such as unsuitability for certain electron-rich hetercoycles and electron-deficient sugar substrates, still exist, we believe this work will stimulate more research in the radical N-glycosylation for the preparation of valuable N-glycosides that are difficult-to-made in future.

Methods
For 1 H, 13 C, and 19 F nuclear magnetic resonance (NMR) spectra of compounds in this manuscript and details of the synthetic procedures as well as more reaction condition screening, see Supplementary Information.
General procedure for 2a-2k, 3d, 3f, 3g, 8l.For more substrate procedures see Supplementary Information An oven-dried 10 mL Schlenk tube was charged with 1-hydroxylmannose 1a (93.6 mg, 0.36 mmol, 1.8 equiv), NHC (142.4 mg, 0.36 mmol, 1.8 equiv) and a magnetic stir bar.After the Schlenk tube was vacuumed and refilled with nitrogen gas three times, dry toluene (2.0 mL) was added and the reaction was stirred at r.t. for 5 min.Then, pyridine (29.1 µL, 0.36 mmol, 1.8 equiv) was added dropwise at room temperature.The resulting solution was stirred at r.t. for 10 min.A white solid precipitated out during this time.Another 10 mL Schlenk tube was charged with 1,2,3,5-Tetrakis(carbazol-9-yl)−4,6-dicyanobenzene (4CzIPN, 5 mg, 0.01 mmol, 0.05 equiv), Cu(MeCN) 4 PF 6 (7.4 mg, 0.02 mmol, 0.1 equiv), dtbbpy (4,4-di-tert-butyl bipyridine, 8.0 mg, 0.03 mmol, 0.15 equiv), CsOAc (38.2 mg, 0.2 mmol, 1.0 equiv), N-heterocycle (0.2 mmol, 1.0 equiv) and a magnetic stir bar.This Schlenk tube was vacuumed and refilled with nitrogen gas three times.Dry acetonitrile (2.0 mL) was added to this Schlenk tube under an atmosphere of nitrogen and stirred at room temperature.The toluene suspension was transferred to a 5 mL syringe under an atmosphere of nitrogen.Then a syringe filter and new needle were installed on the syringe, and the toluene solution was injected through the syringe filter into the MeCN solution.t-BuOOH (80 μL, 5-6 M in decane, 2.0 equiv) was added, before subjecting the reaction mixture to irradiation by 420 nm blue LEDs at room temperature for a duration of 4 hours.The organic layers were evaporated and then purified by flash column chromatography on silica gel.

Fig. 1 |
Fig. 1 | The importance of N-glycosides and the background of this research.a Representative examples N-glycosides as marketed drugs and bioactive compounds; b Selected examples of ionic glycosylation; c previous N-glycosylation via radical activation; d this work: dehydroxylative radical N-glycosylation.
conducted on 0.1 mmol scale.b Yields were determined by 1 H NMR, using 3,5-bis(trifluoromethyl)bromobenzene as the internal standard.c Isolated yield.d Isolated yield on 1 mmol scale.